Various algorithms are currently used by present wireless communication systems for estimating channel quality at a wireless receiver. These algorithms are employed, for example, in systems using the Third Generation Partnership Project (3GPP) High Chip Rate Time Division Duplex (TDD) mode, the 3GPP Low Chip Rate TDD mode, the 3GPP Frequency Division Duplex (FDD) mode, the time division—synchronous code division multiple access (TD-SCDMA) standard, and High Speed Downlink Packet Access (HSDPA) extensions of the aforementioned systems. The quality estimates may be used for transmit power control, in- and out-of-synchronization decisions, radio link failure decisions, and channel quality indicators (CQIs) to support dynamic link adaptation, (e.g., adaptive modulation and coding (AMC)) techniques.
In the TDD mode for instance, the quality indicator, referred to as CQI, sent by the User Equipment (UE) on the high speed-shared information channel (HS-SICH) is a recommended Transport Format Resource Combination (TFRC). In general, the TFRC refers to the possible transport block sizes, modulation schemes, and any other link adaptation parameters available. The recommended TFRC is usually based on the signal most recently received by the UE.
Regardless of whether or not the communication system is a 3GPP system, the CQI could represent a recommended Transport Block Size, modulation format, number of codes, power offsets, or any one of a number of different types of link adaptation parameters. These CQIs are derived by a receiver and signaled to a transmitter to set the transmission parameters for a subsequent transmission.
The CQI typically provides either specific link adaptation information, such as a recommended coding and modulation scheme for the AMC function, or provides one or more general quality indicators which are subsequently used to base the selection of appropriate transmission parameters.
If the CQI is not accurate, the selected modulation and coding scheme (or other transmission parameters) will be suboptimal. Overestimating channel quality can cause the UE and Node B to continue attempting to use a modulation and coding scheme when reception quality is too poor to justify their continued use. Underestimation of channel quality may lead to excessive transmission power and inefficient use of radio recourses or, in the case of in- and out-of-sync processing, ultimately a premature declaration of radio link failure and release of radio resources. Thus, a call may be dropped without cause. Excessive transmission power will, in turn, lead to a system-level throughput loss since interference in other cells may increase needlessly. Accordingly, inaccurate channel quality estimation reduces throughput, wastes transmit power, and increases interference to other cells.
A shortcoming of prior art channel estimation techniques is that since the techniques estimate channel quality at a receiver, they do not provide sufficiently accurate estimates of channel quality at the transmitter at the time of the subsequent transmission. Referring to FIG. 1, a prior art CQI generation and reporting procedure 100 between a UE and a Node B is shown. The Node B transmits a message on a downlink (DL) control channel (step 102), informing the UE which resources have been allocated to the UE for the next associated DL data transmission. The UE receives the control message regarding the allocation of resources and awaits the receipt of the DL data transmission (step 104).
The Node B sends the associated DL data transmission (step 106). The UE reads the DL data transmission (step 108) and makes selective quality measurements (step 110). Using the measurements from step 110, the UE derives a CQI (step 112) that it estimates would provide the highest throughput, while still meeting other possibly specified requirements, such as a block error rate (BLER).
The UE then reports the most recently derived CQI to the Node B in the next available UL control channel (step 114). The Node B receives the CQI (step 116) and then uses the CQI to set the transmission parameters for the next data transmission (step 118).
There are drawbacks with the current method of providing CQI feedback. For example, the current 3GPP specification does not set a specific time limit on how long the UE may take to derive the CQI. This could take an inordinately long time. It is, however, required (and desirable) that once the CQI is derived from the given data transmission, it is reported in the next available UL control channel. This minimizes the delay in getting the CQI information to the Node B. However, even if the delay in getting the CQI information from the UE to the Node B is minimized, the delay is not eliminated.
As shown in the example timing diagram of FIG. 2, there is a CQI measurement period on one or more DL transmissions, during which the UE makes selective measurements on the DL transmission. As shown, the measurements may be performed on a DL data channel, a DL pilot channel, or a combination of both the DL data and pilot channels. After the measurements are performed, the CQI is calculated; this is shown at time t1. Although the delay is minimized by reporting the CQI to the Node B at the next available UL transmission (shown at time t2), there is additional delay until the subsequent use by the Node B of the CQI (shown at time t3) to set the parameters for the next downlink data transmission.
The delay (graphically designated as A) between the completion of the measurements upon which the CQI is based (at time t1) and the subsequent use by the Node B to set the associated transmission parameters at time t3 results in a CQI that is not accurate by the time it is used by the Node B. The greater this delay, the less accurate the CQI becomes. As the CQI becomes less accurate, the DL channel quality will ultimately suffer since the transmission parameters will be based on a CQI that does not accurately reflect the true channel conditions. In essence, the prior art methods of CQI determination reflect the past conditions of the channel.
It would be desirable to provide a method of channel quality determination without the severe disadvantages of known prior art systems.